Features
ELM327 USB is the latest PC-based scan tool. It supports all OBD-II protocols and is dispatched with a number of compatible programs. The processor is an ELM327 USB. The output protocol (connection to laptop) is USB.

Processor:             ELM327 USB
OBD-II Protocols:   ISO15765-4 (CAN)
                              ISO14230-4 (KWP2000)
                              ISO9141-2
                              J1850 VPW
                              J1850 PWM
Output protocol:     RS232
Baud rate:              9600 or 38400
Indicator LEDs:       OBD Tx/Rx, RS232 Tx/Rx, Power
Operating voltage: 12V, internal protection from short circuits / overvoltages
Nominal idle current: 45 mA

Description:

OBD-II Software for ELM327 USB is a free program that allows you to use your PC and a hardware interface to get the information from your car's computer.
The program is very user-friendly, and easy to learn. It is also very easy to install: simply extract the files into a folder on your computer's hard drive, and you're ready to go. To uninstall the program, simply delete the entire contents of the folder where the program resides.
The software supports two platforms - DOS and Windows.

Features:
The program lets you perform the following operations:
Read diagnostic trouble codes, both generic and manufacturer-specific, and display their meaning (over 3000 generic code definitions in the database).
Clear trouble codes and turn off the MIL ("Check Engine" light)
Display current sensor data, including:
             Engine RPM
             Calculated Load Value
             Coolant Temperature
             Vehicle Speed
             Short Term Fuel Trim
             Long Term Fuel Trim
             Intake Manifold Pressure
             Timing Advance
             Intake Air Temperature
             Air Flow Rate
             Absolute Throttle Position
            Oxygen sensor voltages/associated short term fuel trims
            Fuel System status
            Fuel Pressure
            Many others...

Upcoming Features:
            Data graphing and logging
            Freeze Frame data
            Continuous and Non-Continuous Oxygen Sensor test results

Free software:  

            Scantool.net:
            GM Mode 22 Scan Tool by Terry
            OBD Gauge for PalmOS and Pocket PC by Dana Peters
            OBD Logger by Jonathan Senkerik
            OBD-II ScanMaster by Wladimir Gurskij (ScanMaster 3.52 - local copy)
            obd2crazy.com:
            OBD2 Scantool by Ivan Andrewjeski
            OBDII for ELM322 by David Huffman
            pyOBD by Donour Sizemore for MacOSX and Linux
            RDDTC by Pete Calinski
            Real Scan by Brent Harris
            ScanTest for Pocket PC by Ivan Ganev aka a-ser
            wOBD by WDT